In recent years, in cellular mobile communication systems, transmission of large volumes of data such as still image data and moving-image data as well as audio data has been generalized with accompanying the increase of multimedia information. Techniques for realizing a high-transmission rate using a high-frequency wireless band have been actively studied to realize the transmission of large volumes of data.
When the high-frequency wireless band is used, however, a high-transmission rate is expected over a short distance, but attenuation increases as the transmission distance increases. Accordingly, when a mobile communication system using a high-frequency wireless band is actually operated, a coverage area of a wireless communication base station apparatus (hereinafter, simply referred to as a base station) is reduced. Therefore, it is necessary to install more base stations. Since the installation cost of base stations is considerable, techniques are required to realize communication services using a high-frequency wireless band while reducing the number of base stations.
For such a demand, in order to expand a coverage area of each base station, as shown in a wireless relay system in FIGS. 27(a) and 27(b), a relay transmission technique is considered, in which a wireless communication relay station device (Relay Node; RN) 20 (hereinafter referred to as “relay station” 20) is installed between a base station (evolved Node B; eNB) 10 and a wireless communication mobile station device (User Equipment; UE) 30 (hereinafter referred to as “mobile station” 30), and communication between the base station 10 and the mobile station 30 is performed through the relay station 20. Using the relay technique, even the mobile station, which is unable to directly communicate with the base station 10, can communicate through, the relay station.
[Explanation of TD Relay]
LTE-A (Long Term Evolution Advanced) system is demanded to maintain compatibility with LTE from the viewpoint of a smooth shift from LTE and coexistence with LTE. Accordingly, even with respect to a relay technique, it is demanded to achieve mutual compatibility with LTE. In the LTE-A system, in order to achieve the mutual compatibility with LTE, it is considered to set an MBSFN (Multicast/Broadcast over Single Frequency Network) subframe during transmission from a base station to a relay station in a downlink (hereinafter referred to as “DL”).
Next, referring to FIG. 27(a), a TD relay will be described. FIG. 27(a) is a conceptual diagram explaining a TD relay in a downlink, and FIG. 27(b) is a conceptual view explaining a Td relay in an uplink. The TD relay (which is called a half duplex relay or type 1 relay) divides by time transmission from the base station 10 to the relay station 20 and transmission from the relay station 20 to the mobile station 30.
In the uplink illustrated in FIG. 27(b), in subframe #2, transmission from the mobile station 30 to the relay station 20 is performed by an access link, and in subframe #3, communication from the relay station 20 to the base station 10 is performed by a backhaul link. In subframe #4, transmission from the mobile station 30 to the relay station 20 is performed again. In the same manner, in the downlink illustrated in FIG. 27(a), in subframe #2, transmission from the relay station 20 to the mobile station 30 is performed by an access link, and in subframe #3, communication from the base station 10 to the relay station 20 is performed by a backhaul link. In subframe #4, transmission from the relay station 20 to the mobile station 30 is performed again.
As described above, by dividing the communication of the backhaul and the communication of the access link of the relay station 20 by time domain, the time when the relay station 20 performs transmission and the time when the relation 20 performs reception can be divided. Accordingly, the relay station 20 can relay signals without being affected by temporal difference between a transmission antenna and a reception antenna.
Further, in the downlink, an MBSFN subframe is set in an access link. The “MBSFN subframe” is a subframe that is defined to transmit MBMS (Multimedia Broadcast Multicast Service) data. The operation of an LTE terminal is determined so that the LTE terminal does not use a reference signal in the MBSFN subframe.
Accordingly, in LTE-A, a technique has been proposed to avoid an erroneous detection of the reference signal, of an LTE terminal by setting the subframe of the access link side of the relay station cell to the MBSFN subframe in the subframe for the backhaul link in which the relay station 20 communicates with the base station 10. FIG. 28 is a diagram illustrating an example of allocation of a control signal and data in the subframe in each station in the LTE system. As shown in FIG. 28, in the LTE system, the control signal of each station is arranged in front of the subframe. Due to this, the relay station 20 must transmit a control signal (PDCCH: Physical Downlink Control Channel) portion to the mobile station 30 even in the MBSFN subframe. The relay station 20 switches to reception of the backhaul link after transmitting the control signal to the mobile station 30 by the access link, to receive the signal from the base station 10. Accordingly, the relay station 20 is unable to receive the control signal transmitted by the base station 10 while the base station 10 transmits the control signal, to the mobile station 30 by the access link. Due to this, in the LTE-A, it is considered to newly arrange the control signal for the relay station (R-PDCCH) in the data area.
[Explanation of a Control Signal]
The control signal of the LTE system is transmitted from the base station to the mobile station using, for example, a downlink control channel such as PDCCH (Physical Downlink Control Channel). PDCCH includes a “DL grant” that instructs data allocation of DL, and “UL grant” that instructs data allocation of uplink (hereinafter also referred to as “UL”).
As described in Non-Patent Literature 1, it is considered to arrange “DL grant” at a first slot as the control signal for a relay station (R-PDCCH), and to arrange “UL grant” at a second slot. By arranging “DL grant” only at the first slot as the control signal for a relay station (R-PDCCH), decoding delay of “DL grant” can be shortened, and the relay station can prepare for the transmission (after four subframes in FDD) of ACK/NACK for DL data.
Further, it is considered that PRB (Physical Resource Block) in which R-PDCCH is arranged differs for each relay station. FIG. 29 shows an arrangement example of R-PDCCH. The vertical axis of FIG. 29 represents frequency and the horizontal axis of FIG. 29 represents time. As illustrated in FIG. 29, in the same subframe, R-PDCCH for relay station RN1 is arranged in PRB #1, and R-PDCCH for another relay station RN2 is arranged in PRB #6, 7.
Further, the relay station 20 performs blind decoding of R-PDCCH in a search space of R-PDCCH that is instructed as “higher layer signaling” from the base station 10.
[Aggregation Size of R-PDCCH]
In the same manner as PDCCH of the LTE system, it is considered to prepare plural aggregation sizes in order to change the encoding rate of “DL grant” and “UL grant” by channel quality. FIG. 30 is a conceptual diagram illustrating aggregation size of R-PDCCH. In the drawing of the aggregation size of each R-PDCCH, the vertical, axis represents frequency, and the horizontal axis represents time. As shown in FIG. 30, since the encoding rate becomes heightened as the aggregation size becomes smaller, such as 8, 4, 2, 1, R-PDCCH having a small aggregation size is suitable to a case where channel quality between the base station 10 and the relay station 20 is good.
The base station 10 determines the aggregation size of R-PDCCH by estimating the channel quality between the own station and the relay station 20, and transmits R-PDCCH that is generated based on the determined aggregation size to the relay station 20. Since the relay station 20 does not know the aggregation size that is changed every subframe in advance, it performs blind decoding of R-PDCCH in plural aggregation sizes.
[Arrangement of DM-RS]
In LTE-A, DM-RS (Demodulation Reference Signal) that is used for channel estimation is arranged on two rear symbols of each slot. FIG. 31 shows the arrangement example of DM-RS. The vertical axis of FIG. 31 represents frequency and the horizontal axis of FIG. 31 represents time. As shown in FIG. 31, in the case of a normal, subframe, DM-RS (inscribed, as DM-RS port 7, 8 in FIG. 31) is arranged in OFDM symbol #5, #6 and OFDM symbol #11, #12.